A. Technical Field
The present invention relates to a water-absorbent resin powder and its production process and use.
B. Background Art
In recent years, water-absorbent resins (water-absorbing agents) are widely used as constituent materials of sanitary materials, such as disposable diapers, sanitary napkins, and so-called incontinent pads, for the purpose of causing the water-absorbent resins to absorb body fluids.
Known examples of the above water-absorbent resins are as follows: crosslinked polymers of partially neutralized polyacrylic acids; hydrolyzed products of starch-acrylic acid grafted polymers; saponified products of vinyl acetate-acrylic acid ester copolymers; hydrolyzed products of acrylonitrile- or acrylamide copolymers, and their crosslinked polymers; and crosslinked polymers of cationic monomers.
It is said that the above water-absorbent resins should, for example, have the following properties: upon contact with aqueous liquids such as body fluids, excellent water absorption quantity and rate, the gel strength, the suction power to suck up water from a base material containing aqueous liquids. In addition, a variety of water-absorbent resins or a variety of absorbent structures or articles using the water-absorbent resins are proposed, wherein the water-absorbent resins jointly have a plurality of the above properties and exhibit excellent performance (absorption properties) when used for sanitary materials such as disposable diapers and sanitary napkins.
Known examples of the above water-absorbent resins or absorbent structures or articles using the water-absorbent resins are as follows: a water-absorbent resin comprising combinations of a gel volume, a shear modulus, and an extractable polymer content as are specified (U.S. Pat. No. 4,654,039); a water-absorbent resin with a water absorption quantity, a water absorption rate, and a gel strength as are specified, and disposable diapers and sanitary napkins using this water-absorbent resin (JP-A-60-185550, JP-A-60-185551, JP-A-60-185804, and U.S. Pat. No. 4,666,975); disposable diapers using a water-absorbent resin having a specific water absorption quantity, a specific water absorption rate, and a gel stability (JP-A-60-185805); water-absorbent articles using a water-absorbent resin with a water absorption quantity, a suction power, and a water-soluble content as are specified (JP-A-63-021902); water-absorbent sanitary articles containing a water-absorbent resin with a water absorption quantity, a water absorption quantity under a load, and a gel fracture strength as are specified (JP-A-63-099861); disposable diapers containing a water-absorbent resin with a water absorption quantity and a water absorption rate under a load as are specified (JP-A-02-034167); a water-absorbent composite containing a water-absorbent resin with a water absorbency under a load (AUL) and a particle diameter as are specified (EP 339,461); an absorbent structure containing 60xcx9c100 weight % of a specific water-absorbent resin with a free swell rate (FSR) and a water absorbency under a load in 5 minutes as are specified (EP 443,627); a water-absorbent composite containing at least 30 weight % of water-absorbent resin with a deformation under a load (DUL) and a wicking index (WI) as are specified (EP 532,002); an absorbent article using 30xcx9c100 weight % of a water-absorbent resin with a pressure absorbency index (PAI) and a 16-hour extractability level as are regulated (EP 615,735); a water-absorbent composite of super-absorbent material with a specific water retention capacity, a specific water absorption rate, and a specific liquid permeability under a pressure (U.S. Pat. No. 5,985,944); a water-absorbing agent having a specific diffusing absorbency under pressure (U.S. Pat. No. 5,760,080); a water-absorbing composition having a specific diffusing absorbency index under pressure (EP 761,241); and an absorbent structure using a hydrogel that is, for example, characterized by performance under pressure (PUP) and physiological saline flow conductivity (SFC) (U.S. Pat. No. 5,562,646). In addition, water-absorbent resin powders have problems in that they undergo deterioration of the physical properties due to surface fracture when (air-)transported or incorporated into absorbent articles in process of production or thereafter. However, a water-absorbent resin which undergoes such deterioration of the physical properties little and has excellent impact resistance is also known (EP 812,873).
The properties relating to water absorbency (e.g. water absorption capacity under no load, water absorption capacity under a load) of absorbent structures or absorbent articles containing them, for which water-absorbent resins are used, are surely enhanced by adding the above various improvements to water-absorbent resins when compared with the unimproved properties, and thus such improvements are applied to superabsorbent disposable diapers and so on. However, the liquid permeability between particles of a gel resultant from water absorption of the resin, which is important for practical daily use, is still at an insufficient level, so it is difficult to say that the whole water-absorbent resin, which is contained in the absorbent article, is sufficiently made use of. Furthermore, in the case where an attempt is made to ensure the liquid permeability of the gel under a high load (e.g. 0.3 psi (2.07 kPa) corresponding to an infant""s load), the crosslinking density of the gel needs to be increased at the sacrifice of the water absorbency.
An object of the present invention is to provide: a water-absorbent resin powder and its production process, wherein the water-absorbent resin powder has high liquid permeability under a load, and further, high water absorbency both under a load and under no load; and further, an absorbent structure and an absorbent article, for which this water-absorbent resin powder is used. Another object of the present invention is to provide a water-absorbent resin powder which undergoes little deterioration of the physical properties and has excellent impact resistance when (air-)transported or incorporated into absorbent articles in process of production or thereafter.
The present inventors diligently studied to achieve the above object. As a result, they thought that a cause of insufficient liquid permeability of conventional water absorbent resins was that because particles of such resins have an arbitrarily pulverized shape and therefore have angular or frilly portions in their surfaces, liquid-permeable spaces between particles are non-uniform in absorbent structures or articles which usually include from tens of thousands up to more than hundreds of thousands of the resin particles. The inventors further thought that the above angular or frilly portions of the particle surfaces are so weak as to deteriorate the impact resistance.
Then, it occurred to the inventors to grind the surfaces of the resin particles and then crosslink the surfaces of the resultant particles as a means of eliminating the above cause, and actually, the inventors ground the surfaces of the resin particles and then crosslinked the surfaces of the resultant particles, with the result that the inventors found that while the water absorbency was retained at a conventional level, the liquid permeability under a load was greatly enhanced when compared with conventional cases. In addition, the inventors found that a water-absorbent resin powder resultant from the surface grinding was the water-absorbent resin powder which underwent little deterioration of the physical properties and had excellent impact resistance when (air-)transported or incorporated into absorbent articles in process of production or thereafter. Furthermore, the inventors found that if the above process was used, a novel water absorbent resin powder having high water absorbency and high bulk density was obtainable. In addition, the inventors found that a water-absorbent resin powder, having the arbitrarily pulverized shape and a high bulk density, surprisingly had a rather enhanced liquid permeability between particles under a load in spite of having only a small quantity of spaces when packed. Thus, the present invention was completed.
That is to say, an arbitrarily pulverized water-absorbent resin powder, according to the present invention, is characterized by having a bulk density of not lower than 0.74 (g/ml) and a water absorption capacity of not lower than 20 (g/g) for 0.9 weight % physiological saline under a load of 0.7 psi (4.83 kPa).
An absorbent structure, according to the present invention, is characterized by comprising the above arbitrarily pulverized water-absorbent resin powder according to the present invention and a fibrous material.
An absorbent article, according to the present invention, is characterized by comprising an absorbent layer including the above absorbent structure according to the present invention.
Another absorbent article, according to the present invention, is characterized by being a diaper that comprises an absorbent layer including the above absorbent structure according to the present invention, wherein the absorbent structure has an arbitrarily pulverized water-absorbent resin powder content of not lower than 30 weight %.
A production process for an arbitrarily pulverized water-absorbent resin powder, according to the present invention, comprises the step of obtaining water-absorbent crosslinked polymer particles by way of an aqueous solution polymerization step, and is characterized by further comprising the step of grinding the resultant crosslinked polymer particles until the bulk density thereof increases to not lower than 0.72 (g/ml).
These and other objects and the advantages of the present invention will be more fully apparent from the following detailed disclosure.